Very strange problem. I can load the mag to any number and the gun will fail to completely cycle the first round. Manually cycle the action and every other round fires as normal. Doesn't matter suppressed or unsuppressed. Doesnt matter how many rounds I load. Doesnt matter what magazine. First round also has very little recoil.

What model FiveseveN do you have and how old is it? I've got a USG I bought used that has intermittent ejection issues...at first I thought it was either the magazine spring or catch groove (the groove is a little worn) and I just realized the gun itself has a polymer magazine catch that is worn down. I read a few other forums and I think this might have been enough of an issue that they upgraded the catch to a metal one like in most other pistols. Understandably so. Even my S&W SD9 has a metal magazine catch inside the polymer handle, and that gun was a small fraction of the price. I'd check that and the fit of the magazine to make sure its seating all the way. You can always set the gun down on something and give it a little downward pressure on the mag and shoot a few, so see if that fixes your problem. If you do end up needing a magazine catch you'll have to contact FN, haven't found anything like that anywhere else.